For many years it was believed that thermal evaporation methods were the only methods suitable for economically depositing optical interference layer systems for mirror coatings, anti-reflection coatings and the like. While for many applications sputtering methods were believed to be capable of producing layers of better quality than evaporated layers, they were generally regarded as slow, even after the development of the magnetron sputtering-cathode promised a potential ten-fold increase in then-existing sputter deposition rates.
In recent years, however, sufficient progress has been made in sputtering technology that it has become possible to economically deposit relatively complex, multilayer, optical-interference layer systems, by DC reactive sputtering methods, in large area in-line coating apparatus. In applications where there is sufficient demand for a particular coating type to justify producing the coating in large volumes, for example, volumes totaling about one-million square feet per year or more, it is now possible to produce that coating, by DC reactive sputtering, more economically than is possible using traditional thermal evaporation technology.
The linear magnetron sputtering-cathode has been responsible for providing a basis for the developments that have been made in large scale sputtering technology. In a linear magnetron cathode a magnetic array forms a closed-loop tunnel-field over a sputtering-cathode target. A gas discharge provided by the sputtering process provides electrons, some of which are trapped in the tunnel-field significantly increasing their chances of collision with sputtering gas molecules. This creates a highly-ionized plasma in the tunnel-field area. As a result of this highly-ionized plasma, sputtering rates in the region of the plasma are increased, and, even though the plasma does not cover the entire target area, the deposition rate for a magnetron sputtering-cathode may be about ten times the deposition rate for a non-magnetron cathode having the same area.
During the development of sputtering processes many sophisticated developments directed at improving sputtering uniformity were made. These developments were made by concentrating on areas such as sputtering-gas distribution and anode design, for controlling plasma potential uniformity and stability. As far as increased deposition rates are concerned, however, developments have concentrated simply on finding ways to apply more power to a given target area and to increase the area of the target affected by the race-track-shaped plasma, i.e, increase target utilization.
One solution to the power and utilization problem has been presented by the development of the rotating cylindrical magnetron, wherein the target is of tubular form and rotates about magnetic means forming the tunnel-field. This provides for target utilization of about eighty percent or more, and permits more power to be applied to the target, due to more efficient cooling.
It is emphasized here, however, that, despite the development of sputtering devices such as the rotating cylindrical magnetron, the usual approach to increasing sputter deposition rates is to simply apply as much power as is physically possible to a sputtering device. The limit is usually dictated by how effectively the device can be cooled, generally by flowing water through the device.
A large in-line sputtering device for sputtering architectural glass coatings may consume up to fifty-thousand Kilowatt Hours (50,000 KWH) of electricity in a twenty-four hour period, while depositing only about four-hundred cubic centimeters of coating material. Clearly, this is an inefficient use of power. Only a fraction of the power applied is used to create ions which participate in the sputtering process. The remaining power generates heat which must be removed by the water cooling. The process thus places great demand on precious energy resources and water resources alike.
A further problem, inherent in the DC reactive sputtering process, which has not been solved by any of the recent developments, is that of sputtering-rate control. The problem is as follows.
Typically, in a DC reactive sputtering process, a sputtering gas is supplied which is a mixture of an inert gas and a reactive gas. The reactive gas is usually present in proportions of between about five and twenty-five percent of the gas mixture. If a reactive gas were not present in the mixture, a metal sputtering-target would deposit only metal on a substrate being coated. At a given driving power, and at a given total gas flow-rate, as the percentage of reactive gas is increased, a point is reached where metal deposited on the substrate reacts with the reactive gas to form a compound of the metal and the reactive gas. As the percentage of reactive gas is increased beyond this point, the reactive gas reacts with the metal target to form a layer of the compound on the target. If the reactive gas is oxygen, an oxide is formed on the target. If the oxide is an insulator, positive charge will build up on the target inhibiting further sputtering.
For most metals, the permissible range of oxygen percentage in the sputtering gas, from the onset of oxide deposition on the substrate to the point where target sputtering drops to a near zero rate, is relatively small. For aluminum, for example, this may be only about five percent. If power is increased or decreased, either the gas flow-rate, gas mixture, or both must be changed to establish an optimum deposition condition.
In an in-line sputtering apparatus, substrates to be coated are typically transported at a constant rate past cathodes which are depositing materials of a layer system. Thickness of a given layer in the system is determined by the number of cathodes used to deposit the material of that layer, and the conditions of power, gas flow-rate, and gas mixture composition under which the cathodes are operated. If a layer thickness adjustment is required, it is not unusual to have to adjust all parameters until a new stable deposition point is established. This may take, at a minimum, several minutes, during which time the in-line machine may produce more defective product than a small commercial batch coater would produce good product in a whole day.
Clearly, if in-line coating technology is to be further advanced and more widely applied, a deposition source must be developed which is significantly less wasteful of energy and other natural resources, and in which deposition rate is more readily and responsively adjusted.